KR101819359B1 - Precipitation hardening steel sheet having low directional deviation of mechanical properties and method for manufacturing same - Google Patents

Abstract

The present invention relates to a precipitation-hardening steel sheet and a manufacturing method thereof. The precipitation-hardening steel sheet comprises 0.06-0.15 wt% of C, 1.5 wt% or lower (excluding 0 wt%) of Mn, 0.002-0.03 wt% of P, 0.01 wt% or lower of S, 0.01 wt% or lower of N, 0.3 wt% or lower of Si, 0.01-0.06 wt% of acid soluble Al, 0.03-0.1 wt% of Ti, and the remainder consisting of Fe and inevitable impurities, and has a composite structure of ferrite and pearlite. N_P defined by following mathematical equation 1 is 70-130. [Mathematical equation 1] N_P=(N_X/N_Y)100 (N_X means the number of Ti-based precipitates of a diameter of 10 nm or lower having an arbitrary point in a width direction of the steel sheet as a center and existing in a rectangle with an area of 200 mm50 mm whose long edge is placed in a rolling direction, and N_Y means the number of Ti-based precipitates of a diameter of 10 nm or lower having an arbitrary point in the width direction of the steel sheet as a center and existing in a rectangle with an area of 200 mm50 mm whose long edge is placed in a direction perpendicular to the rolling direction).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precipitation hardening type steel sheet,

The present invention relates to a precipitation hardening type steel sheet and a manufacturing method thereof, and more particularly, to a precipitation hardening type steel sheet which can be suitably applied to structural members such as automobile members and the like and has a small variation in directional material.

In order to improve the impact resistance of a vehicle body as the regulation of impact stability of automobiles is strengthened, a precipitation strengthening type steel sheet is widely used for structural members such as a member, a beam, and a pillar.

The precipitation strengthening method utilizes a phenomenon in which a solution treatment is carried out at a high temperature and then a large number of fine precipitates are formed during cooling to be strengthened by the stress field around the precipitate. The strength of the selective strengthened steel sheet is determined by the tensile strength (TS) Yield Strength (YS)), that is, yield ratio (YS / TS) is high.

Patent literatures 1 and 2 are representative techniques of such precipitation hardening type steel sheets. However, these techniques are advantageous in the yield ratio due to the presence of fine precipitates in the steel. However, the distribution of these fine precipitates is not uniform, and the uniformity of the material is poor in directions.

Korean Patent Publication No. 10-2004-0027981 Korean Patent Laid-Open No. 10-2011-0125860

One of the objects of the present invention is to provide a precipitation hardening type steel sheet having a small variation in material for each direction and a method for manufacturing the same.

An aspect of the present invention is a cemented carbide comprising 0.06 to 0.15% of C by weight. And the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Mn is not more than 1.5% (excluding 0%), P is 0.002 to 0.03% %, The balance Fe, and unavoidable impurities, and having a composite structure of ferrite and pearlite, and having an N _P of 70 to 130 as defined by the following formula (1).

[Equation 1]

N _P = (N _X / N _Y ) x 100

(Where, N _X is the number of Ti-based precipitates having a diameter of 10nm or less present in the rectangle in the center to any point in the width direction center of the steel plate, and the area of the long side is placed in the rolling direction of 200mm × 50mm, and N _Y Refers to the number of Ti-based precipitates having a diameter of 10 nm or less and present in a rectangular area having an area of 200 mm x 50 mm, whose long side is placed in a direction perpendicular to the rolling direction, with an arbitrary point in the center in the width direction of the steel sheet as a center)

According to another aspect of the present invention, there is provided a steel sheet comprising: 0.06 to 0.15% of C; And the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Mn is not more than 1.5% (excluding 0%), P is 0.002 to 0.03% %, The remainder Fe and unavoidable impurities is hot rolled to obtain a hot rolled steel sheet having a finish rolling temperature of Ar3 or higher, winding the hot rolled steel sheet at a coiling temperature exceeding 450 캜 and 700 캜 or less, Cold rolling a cold rolled steel sheet at a temperature of 760 to 850 DEG C under a condition of a rolling reduction of 40 to 75% and a final roll pressure of 5 to 20%, continuously annealing the cold rolled steel sheet at an annealing temperature of 760 to 850 DEG C, And cooling the continuously annealed cold-rolled steel sheet at an average cooling rate of 7 ° C / sec or less (excluding 0 ° C / sec) from the annealing temperature to (Ac1-50) ° C .

As one of various effects of the present invention, the precipitation hardening type steel sheet according to the present invention has a small variation in material in each direction and can be preferably applied to structural members such as automobile members.

1 is a TEM (Transmission Electron Microscope) image of Inventive Example 1. Fig.

Hereinafter, a precipitation hardening type steel sheet having a small material deviation in each direction, which is one aspect of the present invention, will be described in detail.

First, the alloy components and preferable content ranges of the precipitation hardening type steel sheet will be described in detail. It is to be noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.06 to 0.15%

Carbon is a precipitate-forming element and contributes to the improvement of steel strength. When the content of carbon is too low, it is difficult to secure adequate pearlite required for strengthening the grain boundaries, which may result in deterioration of hole expandability, and there is a fear that the strength and yield ratio may be lowered due to insufficient fine Ti precipitates . On the other hand, if the content is excessively high, inclusion segregation zones are formed in the steelmaking process, which may increase the possibility of break out. In addition, a large amount of un-precipitated dissolved carbon bonds with Fe to form a coarse pearlite having a particle size exceeding 3 탆, which increases the possibility of crack initiation at the time of processing and also deteriorates hole expandability and weldability . In the present invention, the content of carbon is controlled to 0.06 to 0.15%, preferably 0.07 to 0.12%.

Mn: 1.5% or less (excluding 0%)

Manganese is a solid solution strengthening element which not only contributes to the improvement of steel strength but also precipitates S in MnS to inhibit plate breakage and high temperature embrittlement by S during hot rolling. However, if the content thereof is excessive, a Mn band may be formed in the rolling direction of the steel sheet, which may cause processing cracks. In the present invention, the content of manganese is controlled to 1.5% or less, preferably 1.4% or less. On the other hand, in the present invention, the lower limit of the manganese content is not particularly limited, but may be preferably limited to 0.1% from the viewpoint of ensuring adequate strength.

P: 0.005 to 0.03%

Phosphorus is the most advantageous element for improving the strength of steel without greatly deteriorating the formability. However, if the content is excessive, the possibility of occurrence of brittle fracture is remarkably increased, so that the possibility of occurrence of plate breakage of the slab at the time of hot rolling is increased, and the soft-brittle transition temperature is increased due to grain boundary segregation. In the present invention, the phosphorus content is controlled to 0.005 to 0.03%, preferably 0.006 to 0.028%.

S: 0.01% or less, N: 0.01% or less

Sulfur and nitrogen are inevitable impurities present in the steel, and it is desirable to control their content to be as low as possible for good welding characteristics. In the present invention, the contents of sulfur and nitrogen are controlled to 0.01% or less, preferably 0.009% or less, respectively.

Si: not more than 0.3%

Silicon contributes to the improvement of steel strength by solid solution strengthening, but is not intentionally added in the present invention. On the other hand, if the content is excessive, surface scale defects may be caused and the surface properties of the plating may be deteriorated. In the present invention, the silicon content is controlled to 0.3% or less, preferably 0.25% or less.

Acid soluble Al: 0.01 to 0.06%

Acid soluble aluminum is an element added for grain size reduction and deoxidation. If the content is excessively low, it is difficult to produce killed steel in a normal stable state. On the other hand, if the content is excessive, it is advantageous to improve the strength of steel due to the effect of grain refinement. However, The possibility of the occurrence of surface defects on the coated steel sheet is increased and the manufacturing cost is increased. In the present invention, the content of acid-soluble aluminum is controlled to 0.01 to 0.06%.

Ti: 0.03 to 0.1%

Titanium reacts with the solid carbon in hot rolling to precipitate Ti precipitates and contributes greatly to the improvement of the strength of steel. If the content is excessively low, it may be difficult to secure adequate strength. On the other hand, if the content is too high, the possibility of cracking of the cast steel in the steelmaking process increases, the production cost increases, . In the present invention, the content of titanium is controlled to 0.03 to 0.1%, preferably 0.04 to 0.08%.

And the balance Fe and inevitable impurities. However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically referred to in this specification, as they are known to one of ordinary skill in the art. In addition, addition of an effective component other than the above-mentioned composition is not excluded, and in particular, the following components may be further included to further improve the mechanical properties of the steel sheet.

B: 0.003% or less (excluding 0%)

Boron suppresses secondary machining brittleness due to P in the steel. However, if the content is excessive, the ductility of the steel sheet may be lowered. In the present invention, the content of boron is controlled to 0.003% or less, preferably 0.002% or less.

0.005 to 0.1% of Nb and 0.005 to 0.1% of V,

Niobium and vanadium contribute to the improvement of steel strength by precipitation strengthening like Ti. However, when the content is excessive, not only the economical efficiency is lowered but also the ductility is lowered. In the present invention, the contents of niobium and vanadium are controlled to 0.005 to 0.1%, respectively.

Hereinafter, the microstructure and precipitates of the precipitation hardening type steel sheet will be described in detail.

The precipitation hardening type steel sheet of the present invention has a composite structure of ferrite and pearlite and is preferably a ferrite based material and contains 2 to 20 percent by area of pearlite can do. If the pearlite content is less than 2% by area, it may be difficult to secure the hole expandability. On the other hand, if the pearlite content is more than 20% by area, there is a high possibility of crack initiation during processing, There is a fear that it will be exacerbated.

In the precipitation hardening type steel sheet of the present invention, N _P defined by the following formula (1) may have a value of 70 to 130. If N _P deviates from the following range, the material deviation per direction may become high, which may cause cracks during machining or drastically deteriorate hole expandability. According to the present invention, by controlling the N _P in the following range, the Hole Expansion Ratio (HER) can be ensured to be 50% or more, and the yield strength material deviation per direction (0 ° , 45 °, and 90 °, the difference between the maximum value and the minimum value of the yield strength is measured to be 40 MPa or less. Thus, excellent workability can be secured.

N _P = (N _X / N _Y ) x 100

(Where N _X means the number of Ti precipitates having a circle-equivalent diameter of 10 nm or less existing in a rectangle having an area of 200 mm x 50 mm placed on the long side in a rolling direction with an arbitrary point in the center in the width direction of the steel sheet as a center, N _Y means the number of Ti precipitates having a circle equivalent diameter of 10 nm or less existing in a rectangular area having an area of 200 mm x 50 mm and a long side placed in a direction perpendicular to the rolling direction with an arbitrary point in the center in the width direction of the steel sheet as a center , The centers of the rectangles for the N _X and N _Y measurements match.)

According to one example, the precipitation hardening type steel sheet of the present invention may have a Q defined by the following formula (2): 1.5 to 28.5. The present inventors intend to improve low-temperature brittleness characteristics by minimizing the P component segregated in the grain boundary. Particularly, since C and P are in competition with each other in the grain boundary segregation degree, C is segregated as much as possible at grain boundaries . If Q is less than 1.5, the P component is segregated to a large extent in the grain boundary, and the DBTT (ductile-brittle transition temperature) characteristic may be deteriorated. On the other hand, if it exceeds 28.5, the carbon content in the steel is excessively high There is a risk of degradation. In the present invention, the Ductile Brittle Transition Temperature (DBTT) can be controlled to -50 ° C or lower by controlling the Q value in the above range, thereby ensuring excellent low-temperature frictional characteristics.

Q = ([C] -0.25 [Ti]) / [P]

(Where each of [C], [Ti] and [P] means the content (weight%) of the corresponding element)

According to one example, the precipitation hardening type steel sheet of the present invention may contain 30 precipitates / μm ² or more of Ti precipitates having a circle equivalent diameter of 10 nm or less. When a large number of fine Ti-based precipitates as described above are formed in the steel, the local stress concentration due to the external impact is suppressed, and the impact resistance characteristic of the steel sheet is improved. On the other hand, in the present invention, Ti-based precipitates may mean TiC single precipitates and / or (Ti, Nb) C complex precipitates.

On the other hand, the higher the number of the fine Ti precipitates per unit area, the more improved the impact resistance characteristics. Therefore, in the present invention, the upper limit of the number of carbides is not particularly limited.

The fine Ti-based precipitates can be formed not only in the ferrite grains but also in the ferrite grain boundaries. The larger the area of the fine Ti-based precipitates formed in the ferrite grains, the more improved the impact resistance of the steel sheet. This is because the carbides existing in the crystal grain significantly interfere with the progress of dislocation during processing, and the yield strength progresses faster than the tensile strength. According to one example, the precipitation hardening type steel sheet of the present invention may have a T value of 85 or more, which is defined by the following formula (3). In the present invention, the yield ratio (YS / TS) of the steel sheet can be secured to 0.75 or more by controlling the T value to the above range. Yield Strength (YS) and Tensile Strength (TS) can be measured by tensile test on the direction perpendicular to the rolling direction with JIS No. 5 test specimens.

T (%) = {T _in / (T _gb + T _in )} x 100

(Where T _in is the total area of the Ti-based precipitates present _in the ferrite grains and T _gb is the total area of the Ti-based precipitates present _in the ferrite grain boundaries)

The precipitation hardening type steel sheet of the present invention described above can be produced by various methods, and the production method thereof is not particularly limited. However, as a preferable example, it can be produced by the following method.

Hereinafter, a method of manufacturing a precipitation hardening type steel sheet having a small material deviation in each direction, which is another aspect of the present invention, will be described in detail.

First, a hot-rolled steel sheet is obtained by hot-rolling a steel slab having the above-mentioned component system to a finish rolling temperature of Ar 3 ° C or higher. The reason why hot rolling is performed in a single phase of austenite is to increase the uniformity of the structure.

Next, the hot-rolled steel sheet is wound.

In order to maximize the effect of increasing the strength by precipitation of the fine Ti precipitates, low-temperature coiling is preferable, but when the coiling temperature is too low, the fine Ti precipitates preferentially precipitate in the ferrite grain boundaries rather than in the ferrite crystal grains, Not only pearlite but also bainite may be formed in the microstructure of the steel sheet, resulting in deterioration of hole expandability. On the other hand, when the coiling temperature is too high, the Ti-based precipitates are coarse, a large amount of Ti-based precipitates are precipitated in the ferrite grain boundaries, and it may be difficult to obtain the desired strength, and coarse pearlite is formed to deteriorate hole expandability . Furthermore, coarse Ti-based precipitates formed by high-temperature coiling are difficult to redissolve during the subsequent annealing process, so that the degree of precipitation in each direction changes, which may deteriorate the material uniformity of the steel sheet, Ti-based precipitates have few sites where the solid solution carbon can remain, so that the solid solution P precipitates on the grain boundaries during annealing, and the DBTT characteristics may deteriorate. Taking this into consideration, the coiling temperature is preferably more than 450 DEG C and not more than 700 DEG C, and more preferably not less than 480 DEG C and not more than 650 DEG C.

According to one example, the average cooling rate from the hot finish rolling temperature to the coiling temperature may be 10-200 占 폚 / sec. If the average cooling rate is less than 10 ° C / sec, the size of the precipitate in the ferrite becomes coarse, and the desired precipitation hardening characteristics of the present invention may not be exhibited. On the other hand, when the cooling rate exceeds 200 ° C / There is a possibility that the temperature of the hot-rolled steel sheet may become uneven and some bainite structure may be generated, which may cause a variation in material in the hot-rolled width direction.

Next, the rolled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.

At this time, the cold reduction ratio is preferably 40 to 75%. If the cold rolling reduction is less than 40%, the number of grain nucleation sites is small, so that not only the Ti precipitates are coarsened upon recrystallization annealing but a large amount of Ti precipitates may precipitate on the ferrite grain boundaries. On the other hand, if it exceeds 75%, not only the rolling load is caused but also the possibility of plate breakage is increased.

According to one example, the final roll pressure ratio during cold rolling may be between 5 and 20%. The reason why the final roll pressure ratio is controlled in the above range is to finely disperse the pearlite segregated in grain boundaries after hot rolling so as to increase the degree of segregation of solid carbon in the annealing process, %, The fine pearlite formed in the hot rolling process can not be easily broken, and the DBTT characteristics may be deteriorated due to the low degree of grain segregation of the solid carbon in the final annealing. On the other hand, if it exceeds 20% There is a risk of plate breakage during cold rolling.

Next, the cold-rolled steel sheet is continuously annealed.

At this time, the annealing temperature is preferably 760 to 850 ° C, and more preferably 780 to 850 ° C. When the annealing temperature is less than 760 DEG C, complete recrystallization does not occur and there is a problem that the deviation in material in the width direction increases. On the other hand, when the temperature exceeds 850 DEG C, precipitates are coarsened and crystal grains grow rapidly, which makes it difficult to secure a desired strength, and there is a problem that the possibility of plate shape failure due to high temperature annealing increases. The present invention more preferably anneals at 790 占 폚 or above to redissolve fine Ti precipitates partially precipitated at hot rolling to induce re-casting to extremely fine Ti precipitates of 10 nm or less during cooling and annealing, To be as uniform as possible so that the yield strength material deviation of the final annealed plate is reduced. In this case, the deviation of the yield strength is specifically controlled because the present invention is applied to automotive parts as precipitation hardened steel, which is high in the yield strength as an impact resistant material. As a result, as annealing at high temperature, some of the coarse Ti-based precipitates precipitated from hot-rolled steel are re-dissolved, and the steel is finely re-precipitated upon cooling after annealing, so that the deviation of the material can be controlled to a low degree.

On the other hand, it is preferable that the annealing time, that is, the holding time at the annealing temperature is secured for 40 seconds or more. When the annealing time is less than 40 seconds, there is not enough time to diffuse from the grain to the grain boundary, and the probability that the P component is segregated at grain boundaries is increased because the Ti precipitates precipitated at the time of hot rolling are partially reused at high temperature annealing As a result, the DBTT characteristic is deteriorated. On the other hand, as the annealing time is longer, the degree of grain boundary C segregation increases, which is advantageous to the DBTT characteristics. Therefore, the upper limit of the annealing time is not particularly limited in the present invention.

Next, the cold-rolled steel sheet subjected to the continuous annealing is cooled to obtain a precipitation hardening type steel sheet.

The average cooling rate from the annealing temperature to (Ac1-50) C may be 7 ° C / sec or less (excluding 0 ° C / sec), preferably 5 ° C / sec or less (excluding 0 ° C / sec) have. This is to suppress as much as possible the generation of fine Ti precipitates at a high temperature as much as possible and the precipitation of residual solid carbon in pearlite or cementite (Fe ₃ C) during cooling. If the average cooling rate exceeds 7 DEG C / sec, sufficient fine Ti precipitates are not generated, and a large amount of pearlite or cementite is formed, which may increase the deviation of material in each direction. On the other hand, the Ac1 temperature can be calculated by the following equation (1).

[Formula 1]

[Ac] (° C) = 723-10.7 [Mn] -16.9 [Ni] +29.1 [Si] +16.9 [Cr] +290 [

Each of [Mn], [Ni], [Si], [Cr], [As] and [W] means the content (weight%

Next, if necessary, the surface of the precipitation hardening type steel sheet may be subjected to hot dip galvanization to obtain a hot dip galvanized steel sheet, or a hot dip galvannealed steel sheet may be subjected to alloying heat treatment to obtain a galvannealed steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

( Example )

A steel slab having the alloy composition shown in the following Table 1 was hot rolled at a finish rolling temperature of 890 캜 and then rolled under the conditions described in Table 2, followed by cold rolling, continuous annealing and cooling to produce a cold rolled steel sheet. The cold-rolled steel sheet thus prepared was measured for carbide distribution, microstructure and mechanical properties, and the results are shown in Table 3 below.

The distribution and area of fine Ti precipitates were determined by image analysis using TEM, and the pearlite area fraction was obtained by image analysis using SEM. 1 is a TEM (Transmission Electron Microscope) image of Inventive Example 1. Fig.

The yield strength material deviation (YS) for each direction was determined by the difference between the maximum and minimum values of the yield strength by tensile test at 0 °, 45 ° and 90 ° using JIS 5 specimen. The ductility-brittle transition temperature (DBTT) was measured according to KS B 0809, and the V-notch was placed in the middle of the test specimens and held at each test temperature for 10 minutes before impact test. The temperature at which a value of 1/2 of the absorbed energy at a temperature at which the ductile wavefront ratio is 100% is taken as the transition temperature.

Steel grade Alloy component (% by weight) Q C Mn P S N Si Ti Nb B Sol.Al Other Inventive Steel 1 0.08 0.2 0.008 0.0046 0.0043 0.15 0.06 - 0.0015 0.035 - 8.1 Invention river 2 0.09 0.4 0.021 0.0056 0.0032 0.22 0.06 - 0.0013 0.032 V: 0.02 3.6 Invention steel 3 0.09 0.8 0.018 0.0085 0.0066 0.23 0.07 0.01 - 0.038 - 4.0 Inventive Steel 4 0.10 1.1 0.025 0.0038 0.0031 0.21 0.08 - - 0.028 - 3.2 Invention steel 5 0.11 1.3 0.024 0.0045 0.0042 0.18 0.05 0.007 0.0008 0.031 - 4.0 Comparative River 1 0.04 2.2 0.025 0.0062 0.0041 0.22 - - - 0.041 - 1.6 Comparative River 2 0.08 2.5 0.015 0.0048 0.0023 0.08 - - 0.0023 0.04 Mo: 0.2 5.3

Steel grade Coiling Cold rolling Annealing Cooling Remarks Temperature
(° C) Average cooling rate
(° C / sec) Reduction rate
(%) Final roll pressure ratio
(%) Temperature (℃) time
(sec) Average cooling rate
(° C / sec) Inventive Steel 1 482 65 62 7.2 782 45 3.8 Inventory 1 482 79 62 7.2 782 43 7.5 Comparative Example 1 563 75 58 7.5 835 43 4.9 Inventory 2 563 68 58 7.5 835 42 7.2 Comparative Example 2 Invention river 2 642 82 62 6.8 783 56 1.2 Inventory 3 742 129 62 5.9 861 55 4.6 Comparative Example 3 Invention steel 3 580 126 63 5.9 775 57 3.8 Honorable 4 563 85 36 2.3 803 51 3.7 Comparative Example 4 Inventive Steel 4 580 81 61 10.2 841 48 4.2 Inventory 5 620 95 81 26 741 49 5.0 Comparative Example 5 Invention steel 5 642 96 61 11.3 792 48 4.8 Inventory 6 620 75 71 9.6 830 68 7.1 Comparative Example 6 Comparative River 1 685 98 62 10.3 793 55 4.7 Comparative Example 7 Comparative River 2 656 89 63 10.5 840 55 4.2 Comparative Example 8 - In the cooling stage, the average cooling rate refers to the average cooling rate from the annealing temperature of each specimen to the temperature (Ac1-50).

Steel grade Microstructure Ti precipitates Mechanical properties Remarks P fraction
(area%) N _P Number density
(Pieces / μm ² ) T YS deviation
(MPa) Yield ratio
(YS / TS) DBTT
(° C) HER
(%) Inventive Steel 1 11.2 88 45 88 18 0.81 -60 62 Inventory 1 10.6 135 23 92 43 0.72 -70 52 Comparative Example 1 12.8 85 51 87 17 0.80 -50 58 Inventory 2 11.5 64 19 95 49 0.68 -65 55 Comparative Example 2 Invention river 2 13.2 82 53 112 32 0.81 -55 56 Inventory 3 14.3 65 55 62 51 0.71 -45 46 Comparative Example 3 Invention steel 3 13.5 78 72 86 25 0.77 -55 57 Honorable 4 12.5 152 76 84 43 0.73 -60 36 Comparative Example 4 Inventive Steel 4 15.2 95 101 86 21 0.81 -60 76 Inventory 5 14.3 145 121 65 52 0.72 -65 36 Comparative Example 5 Invention steel 5 13.8 95 123 86 32 0.79 -65 55 Inventory 6 10.5 68 27 87 67 0.72 -52 52 Comparative Example 6 Comparative River 1 0.3 - 2.5 - 62 0.65 -25 14 Comparative Example 7 Comparative River 2 0.2 - 1.5 - 58 0.62 -35 16 Comparative Example 8 - In the microstructure, P means "pearlite", and all of the examples of the invention are composed of ferrite.
The number density of Ti-based precipitates means the number density of fine Ti-based precipitates having a diameter of 10 nm or less.

Referring to Tables 1 to 3, Examples 1 to 6 satisfying all the alloy compositions and manufacturing conditions proposed in the present invention exhibited a high porosity of 0.75 or more, excellent hole expandability (HER) of 50% or more, (DBTT), and the deviation of the yield strength in each direction is 40 MPa or less, which shows that the material deviation is very small.

However, in the case of Comparative Examples 1 to 5, it was confirmed that either the alloy composition or the manufacturing conditions did not satisfy the conditions proposed in the present invention, and the mechanical properties were poor. In particular, in the case of Comparative Examples 7 and 8, the Mn or C content was excessively high, and after the annealing, the ferrite and the pearlite composite structure were not formed, and a fine martensite structure was formed in the ferrite structure.

Claims (13)

C: 0.06 to 0.15% by weight. And the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Mn is not more than 1.5% (excluding 0%), P is 0.002 to 0.03% %, The balance Fe, and unavoidable impurities,
A composite structure of ferrite and pearlite,
A precipitation hardening type steel sheet having an N _P of 70 to 130 as defined by the following formula (1).
[Equation 1]
N _P = (N _X / N _Y ) x 100
(Where, N _X is the number of Ti-based precipitates having a diameter of 10nm or less present in the rectangle in the center to any point in the width direction center of the steel plate, and the area of the long side is placed in the rolling direction of 200mm × 50mm, and N _Y Refers to the number of Ti-based precipitates having a diameter of 10 nm or less and present in a rectangular area having an area of 200 mm x 50 mm, whose long side is placed in a direction perpendicular to the rolling direction, with an arbitrary point in the center in the width direction of the steel sheet as a center)
The method according to claim 1,
By weight, at least one selected from the group consisting of 0.003% or less (excluding 0%) of B, 0.005 to 0.1% of Nb, and 0.005 to 0.1% of V.
The method according to claim 1,
A precipitation hardening type steel sheet comprising 2 to 20% by area of pearlite.
The method according to claim 1,
A precipitation hardening type steel sheet having a maximum value and a minimum value of 40 MPa or less, when the yield strength is measured by tensile test in the directions of 0 °, 45 ° and 90 ° from the rolling direction with the JIS No. 5 test piece.
The method according to claim 1,
A precipitation hardening type steel sheet having a Q of 1.5 to 28.5 as defined by the following formula (2).
&Quot; (2) "
Q = ([C] -0.25 [Ti]) / [P]
(Where each of [C], [Ti] and [P] means the content (weight%) of the corresponding element)
6. The method of claim 5,
A precipitation hardening type steel sheet having Ductile Brittle Transition Temperature (DBTT) of -50 캜 or lower.
The method according to claim 1,
A precipitation hardening type steel sheet comprising Ti precipitates having a diameter of 10 nm or less at 30 / μm ² or more.
8. The method of claim 7,
A precipitation hardening type steel sheet having a T defined by the following formula (3): 85 or more.
&Quot; (3) "
T (%) = {T _in / (T _gb + T _in )} 100
(Where T _in is the total area of the Ti-based precipitates present _in the ferrite grains and T _gb is the total area of the Ti-based precipitates present _in the ferrite grain boundaries)
9. The method of claim 8,
And a yield ratio (YS / TS) of 0.75 or more.
C: 0.06 to 0.15% by weight. And the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Ti is 0.03 to 0.1%, the content of Mn is not more than 1.5% (excluding 0%), P is 0.002 to 0.03% %, The balance Fe, and unavoidable impurities is hot-rolled to obtain a hot-rolled steel sheet so that the finish rolling temperature is equal to or higher than Ar3;
Winding the hot-rolled steel sheet at a coiling temperature higher than 450 캜 and lower than 700 캜;
Cold rolling the rolled hot-rolled steel sheet at a reduction ratio of 40 to 75% and a final roll-pressing ratio of 5 to 20% to obtain a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet at an annealing temperature of 760 to 850 ° C; And
Cooling the continuously annealed cold rolled steel sheet at an average cooling rate of 7 ° C / sec or less (excluding 0 ° C / sec) from the annealing temperature to (Ac1-50) ° C;
By weight based on the total weight of the steel sheet.
11. The method of claim 10,
Wherein the steel sheet further comprises at least one selected from the group consisting of 0.003% or less of B (excluding 0%), 0.005 to 0.1% of Nb, and 0.005 to 0.1% of V, in terms of% by weight.
11. The method of claim 10,
And the average cooling rate from the finish rolling temperature to the coiling temperature is 10 to 200 占 폚 / sec.
11. The method of claim 10,
Wherein the annealing time in annealing is 40 seconds or more.

Images